Listeria monocytogenes is a Gram-positive, facultatively anaerobic bacterium and the causative agent of listeriosis — a serious foodborne infection with a case fatality rate of approximately 20–30% in high-risk groups (pregnant women, neonates, elderly, and immunocompromised individuals). Unlike most foodborne pathogens, L. monocytogenes can grow at refrigeration temperatures (2–4 °C), making it particularly challenging to control in chilled ready-to-eat foods.
Key characteristics relevant to food safety testing:
- Growth range: 0–45 °C, with optimal growth at 30–37 °C; notably survives and multiplies at refrigeration temperatures.
- High salt tolerance (grows in up to 10% NaCl) and acid tolerance, enabling persistence in processed food environments.
- Found widely in soil, water, animal feed, and food production environments; can form biofilms on food contact surfaces.
- High-risk foods: smoked and cured fish, deli meats, soft cheeses, pâté, and pre-packaged salads.
- Testing is conducted as part of Ovalab’s accredited microbiological analysis service.
Detection and Enumeration Methods
Laboratory testing for Listeria monocytogenes follows two complementary ISO standards:
- EN ISO 11290-1:2017 — Detection method: A two-step enrichment procedure. Primary enrichment in half-Fraser broth (24–26 h at 30 °C), followed by secondary enrichment in full Fraser broth (24–26 h at 37 °C). Selective plating on chromogenic agar (e.g., ALOA) and confirmation by biochemical tests. Duration: approximately 5 working days.
- EN ISO 11290-2:2017 — Enumeration method: Direct plating from the homogenate onto selective media for quantification in colony-forming units per gram (cfu/g). Used when enumeration is required to verify compliance with the 100 cfu/g shelf-life criterion.
- Real-time PCR (PCR-based methods): Molecular screening targeting specific L. monocytogenes genes (e.g., hlyA encoding listeriolysin O). PCR-based kits validated against EN ISO 11290-1 enable screening results within 24–48 hours, significantly reducing turnaround time.
- Environmental monitoring: Surface swabs from food processing equipment, drains, and refrigerated storage areas are regularly tested for Listeria spp. as part of HACCP programmes.
EU Regulatory Limits
Commission Regulation (EC) No 2073/2005 on microbiological criteria for foodstuffs sets the following limits for Listeria monocytogenes:
- Absence in 25 g: Applies to ready-to-eat (RTE) foods intended for infants and for special medical purposes — a zero-tolerance standard throughout the shelf life.
- 100 cfu/g: Applies to RTE foods able to support the growth of L. monocytogenes (foods placed on the market during their shelf life where the manufacturer can demonstrate the limit will not be exceeded). This criterion applies throughout the shelf life.
- Absence in 25 g (at processing stage): Applies to RTE foods able to support L. monocytogenes growth before the product has left the immediate control of the producing food business operator, when no shelf-life growth data is available.
Commission Regulation (EU) 2024/2895 of 20 November 2024 further amended the Listeria criteria in Regulation (EC) No 2073/2005, with new provisions applying from 1 July 2026. Food producers are advised to conduct shelf-life studies in accordance with Annex II of Regulation (EC) No 2073/2005 to establish compliant limits for their specific products.
Frequently Asked Questions
Why is Listeria monocytogenes particularly dangerous?
Listeria monocytogenes is uniquely hazardous among foodborne pathogens because it can grow at refrigeration temperatures (as low as 0–2 °C), meaning chilling does not eliminate the risk. In ready-to-eat products stored for extended shelf lives, even a small initial contamination can reach dangerous levels before consumption. This property explains why L. monocytogenes is a primary concern in cold-stored deli meats, smoked fish, and soft cheeses.
What are the EU limits for Listeria in food?
The genus Listeria contains multiple species, of which L. monocytogenes is the primary human pathogen. Screening for Listeria spp. detects all Listeria species (including the non-pathogenic L. innocua), making it useful for environmental hygiene monitoring. Specific detection of L. monocytogenes is required for food safety criteria compliance and uses confirmation steps (biochemical, serological, or molecular) to differentiate the pathogenic species from others in the genus.
Which foods require Listeria testing?
Under Commission Regulation (EC) No 2073/2005, a zero-tolerance standard (absence in 25 g) throughout the entire shelf life applies to ready-to-eat foods intended for infants and foods for special medical purposes. For other RTE food categories able to support L. monocytogenes growth, the limit is 100 cfu/g throughout shelf life, provided the manufacturer can demonstrate through shelf-life studies that this limit will not be exceeded.
How long does Listeria testing take?
In an accredited laboratory, L. monocytogenes testing follows EN ISO 11290-1:2017 for detection and EN ISO 11290-2:2017 for enumeration. The detection method involves two sequential enrichment steps in selective broths, followed by plating on chromogenic selective agar and biochemical/molecular confirmation. PCR-based screening methods validated against the ISO standard can accelerate initial screening to 24–48 hours. Ovalab’s microbiological analysis service covers both detection and enumeration testing.
What is the difference between L. monocytogenes and Listeria spp.?
Commission Regulation (EU) 2024/2895 of 20 November 2024 amended Regulation (EC) No 2073/2005 with updated Listeria criteria taking effect from 1 July 2026. The revision extended responsibility for Listeria criteria to additional food business operators in the supply chain and clarified the application of limits throughout shelf life. Food producers are advised to review their shelf-life studies and HACCP plans to ensure compliance with the updated requirements before the implementation date.